1. Make sure Rockbox only scans your SDCard (/mnt/external_sd)
2. In settings, the option "Gather Runtime Data" is set to "NO"
Both of them are in Settings -> General Settings -> Database.
If it's still not building, clear the contents of the rockbox folder in the DX90's internal storage.
If all else failed, there there is a file/s rockbox is unable to read in your SDCard.
#1 worked for me. I thought I'd enabled this before but hadn't. Got it right this time and DB built quickly. Thanks for you help!