You have a few options.
Once you add an external hard disk, if you use iTunes, you can "uncheck" the box to keep your music library organised and prevent it from copying new songs you add into the folder you originally designated. This way, when you drag new files to iTunes to add them to the library, it will add the tracks into the library without actually copying the files over. And the files will remain in the external HDD.
Of course, you will still need an app like XLD to convert your FLAC files from HD Tracks to AIFF or ALAC.
While this method will work, I am a bit too OCD to use this. It means all your media files are all over the place and stored in a central place. This will also be important if you want to make back ups of your music library - and yous should since you know HDDs have a finite shelf life.
The method I'd propose involves getting a much larger HDD. It's cheap these days, so I am going to say 4TB. In iTunes, you have an option to specify where your music library is stored in Preferences/Advanced. Instead of user/Music folder, point it to the external HDD - say an iTunes folder in the external HDD. And in this case, you want to "check" the keep your library organised and copy files to the library.
You'll be asked if you want to move your existing library to the new location. Click Yes.
And iTunes will proceed to copy all your files across from your internal HDD to the external drive.
Easy.
Remember I said storage is cheap? Maybe get two of those of external drives. And use Disk Utilty to duplicate your first external drive on the second. Do it periodically.
It's like insurance.
OK. We got the basics covered.
Now what else can you do to improve the playback quality on your iMac?
If you are handy with opening up the machine, I'd recommend getting an SSD and as much memory as your iMac can support.
Replacing a moving array of parts in a HDD with a solid state drive with no moving parts will help the computer work better as a music server, from what I've discovered. I suspect it's all the EMI and stray RF from all the magnets and moving parts.
Loading up the RAM allows you to benefit from the next stage - memory play - where tracks are loaded from the HDD completely first into memory and then played from memory.
To do this, you will need some software. The cheapest I think should be BitPerfect. It's available from the App Store.
BitPerfect does a couple of things in addition to memory play.
A. it allows INTeger mode and an exclusive use of the audio device.
b. it dynamically switches the sample rate to mix the source file you are playing. If you don't do that, Apple's OS will try to resample your music from its original rate (say 44.1kHz from your CD rips) to say 96kHz if that's what is set in Audio MIDI control.
You might think playing at 96kHz should sound better. But in reality, it depends on the algorithms used to upsample. And the Apple OS (as with most OSes) does a shitty job of this.
So keeping it "native" is better. If you do want to upsample, apps like BitPerfect will also do that for you. And it does a better job too.
Before you decide on BitPerfect (it's the cheapest but there's no trial option), you can also run trials of Pure Music (now in v2.0), Audirvana Plus (my recommendation based on price/performance as it does DSD as well) or Amarra.
Once you add an external hard disk, if you use iTunes, you can "uncheck" the box to keep your music library organised and prevent it from copying new songs you add into the folder you originally designated. This way, when you drag new files to iTunes to add them to the library, it will add the tracks into the library without actually copying the files over. And the files will remain in the external HDD.
Of course, you will still need an app like XLD to convert your FLAC files from HD Tracks to AIFF or ALAC.
While this method will work, I am a bit too OCD to use this. It means all your media files are all over the place and stored in a central place. This will also be important if you want to make back ups of your music library - and yous should since you know HDDs have a finite shelf life.
The method I'd propose involves getting a much larger HDD. It's cheap these days, so I am going to say 4TB. In iTunes, you have an option to specify where your music library is stored in Preferences/Advanced. Instead of user/Music folder, point it to the external HDD - say an iTunes folder in the external HDD. And in this case, you want to "check" the keep your library organised and copy files to the library.
You'll be asked if you want to move your existing library to the new location. Click Yes.
And iTunes will proceed to copy all your files across from your internal HDD to the external drive.
Easy.
Remember I said storage is cheap? Maybe get two of those of external drives. And use Disk Utilty to duplicate your first external drive on the second. Do it periodically.
It's like insurance.
OK. We got the basics covered.
Now what else can you do to improve the playback quality on your iMac?
If you are handy with opening up the machine, I'd recommend getting an SSD and as much memory as your iMac can support.
Replacing a moving array of parts in a HDD with a solid state drive with no moving parts will help the computer work better as a music server, from what I've discovered. I suspect it's all the EMI and stray RF from all the magnets and moving parts.
Loading up the RAM allows you to benefit from the next stage - memory play - where tracks are loaded from the HDD completely first into memory and then played from memory.
To do this, you will need some software. The cheapest I think should be BitPerfect. It's available from the App Store.
BitPerfect does a couple of things in addition to memory play.
A. it allows INTeger mode and an exclusive use of the audio device.
b. it dynamically switches the sample rate to mix the source file you are playing. If you don't do that, Apple's OS will try to resample your music from its original rate (say 44.1kHz from your CD rips) to say 96kHz if that's what is set in Audio MIDI control.
You might think playing at 96kHz should sound better. But in reality, it depends on the algorithms used to upsample. And the Apple OS (as with most OSes) does a shitty job of this.
So keeping it "native" is better. If you do want to upsample, apps like BitPerfect will also do that for you. And it does a better job too.
Before you decide on BitPerfect (it's the cheapest but there's no trial option), you can also run trials of Pure Music (now in v2.0), Audirvana Plus (my recommendation based on price/performance as it does DSD as well) or Amarra.