I have a 500GB external HDD which I'm going to test with a mini USB3 Y-cable. The other end has two USB cables which I'm going to plug into both ports on the AVH-4000. If it works, I'll have a solution with an external drive I already have, which will hopefully work until I can budget some big SSDs. I will actually install a separate USB power source ($13) to plug the 2nd USB plug into, a dual USB which would allow two HDDs to connect. If all goes well, I could add a 2nd drive just like it for $60 vs about $230 for an SSD + enclosure. Dang budget. It seems clear that a typical SSD USB drive can simply plug into the back and work. If it does work, it'll probably take quite a while on the first read, but apparently that goes away after the initial run. I don't know if there is any difference in the speed of reading the files initially on an SSD vs HDD.
I have the drive maxed out at 14,999 FLAC files, within 1,421 folders, primarily across-the-board rock music. I don't know how many CDs it is, but it's in the hundreds, as a big percentage of the 1,421 folders represent individual CDs. With a couple exceptions for testing purposes, these FLAC files are all individual songs. This batch of music yields 370GB. The range, compressed, seems to be 200-500MB per CD, with most between 250-350MB per disk. I guess that if I figured 370MB per CD, I've got roughly 1,000 CDs on it. So if you figure 500MB x 500 CDs, you're looking at about 250GB. If they average 300MB per CD, you're at about 150GB. Good chance you can do them all in a 250GB SSD or even a 256GB USB flash drive, or two 128GB USB flash drives.