I installed some cheap generic ZIF SSD into my 5g classic and ran into numerous troubles with getting rockbox to work, namely getting a "dc_writeback...." kernel error that would brick the rockbox OS until I reflashed it. After some searching I found what steps worked for me, I hope it may help solve some current and future head aches as well.
1 - Restore iPod in Itunes, close out of Itunes when it finishes
2 - *Important bit* - Format the main partition as
FAT32, 32KB allocation regardless of drive size. I have a 128gb and I guess the default 16kb of a restore and default 64kb of a format kept screwing me over
3 - Install rockbox, for me a dev version was needed
4 - At this point, transferring a album or w/e to test playback is best. I transferred 40 gigs before I actually tried and got the kernel error on playing a file making me re flash it

5 - Disconnect, cross fingers, it should now
hopefully boot to Rockbox and play files just fine.
note - file transfer only seems to work in the normal iPod bootloader. It may work after the 32kb format but I have not tried it since for fear of potentially bricking and starting over again.....same deal with a non dev version build. It may very well work since the 32kb allocation specific format. But I have a working player now finally loaded with music so alas....
Hope this helps some other peeps!