Ubuntu Phone BQ Aquaris E4.5 USB Problem
Solution 1:
Ubuntu touch does not allow USB interaction as long as the phone screen is locked. Even if it is only locked by slide, this can block the interaction. I also made sure my screen doesn't lock after idleness, so this locking doesn't happen halfway through a transfer.
Also, ~/.android/adb_usb.ini on your computer is a list of valid vendorIDs that are allowed to connect. I found that list empty (or non existent, don't remember, doesn't matter). Just add a new line with the vendorID for your phone: If you have MX4, add 0x2a45, if Aquaris E4.5, the ID is 0x2a47. Otherwise look it up by using lsusb. You can of course also add both to be sure ;)
~/.android/adb_usb.ini :
0x2a45
0x2a47
Solution 2:
Take a tour here to upgrade your MTP library. Hope it could help.
Solution 3:
Just adding my 2 cents because I was also struggling with this and my solution was really weird.
Fresh install of ubuntu 14.04.2 64-bit with all updates installed.
Connected phone with usb 2.0 port: no access (I tried 2 different usb 2.0 ports)
Connected phone with usb 2.0 port and used gMTP (ubuntu software center): access
Connected phone with usb 3.0 port: access (no need for gMTP)