It's been a few days... if you haven't given up yet... I've definetly done this a few times! A few things:
ux0 is the location for vita installed games and homebrew. ux0 must mount at boot to just start playing game bubbles.
Large spin-up drives, also known as 3.5" HDD, can't mount at boot (yet). All other drives (including laptop sized spin-up 2.5" HDDs) that I've tried can mount on ux0 at boot.
If you want to use a 3.5 inch drive (large) then it needs to be mounted using vitashell every time the pstv turns on. All other drives can be used normally.
Both sizes of spin-up HDDs (3.5" and 2.5") require a powered USB hub. Having a powered USB hub is also helpful for charging/using/pairing a controller with a drive connected.
These other drives don't need a powered USB hub:
Flash drives work fine.
USB to SD card adapters work fine.
USB to Sata adaper with a solid state HD works fine.
You can use YAMT or StorageMgr or USBMC, but combining them will cause a boot loop (that we can fix) so choose one. I suggest YAMT because it's much faster and can format the drive for you.
What kind of drive would you like to use?
Which drive mounter would you like to use?
I can help.
Feel free to ask any questions if you don't understand what I wrote.