I think my problem is a little different than several other similar titled questions.
Everything started while I was using 10.10. External drive was always recognized and mounted, but the timing of this differed. I mean when I booted, the OS sometimes saw the drive immediately, sometimes after a few minutes, sometimes after hours. Although this was annoying, I tolerated this somehow.
Then this problem persisted after I upgraded to 11.04 with a clean install. Afterwards, the drive became totally invisible to the OS. It is not even detected as an usb device anymore.
However, there is one thing I can do to make it seen. If I boot to another operating system which can detect the drive, and then boot back to ubuntu, everything is perfect. but this is of course very impractical.
To summarize, the usb drive is recognized by ubuntu if and only if another OS in the same computer could recognize it first. I should also say I have not had any problems with the same drive in any other OS or a different computer.
My drive is philips with a hitachi hdd inside, has its own power source and any other usb powered drives have never caused this kind of a problem.
I've tried almost all recommendations in similar topics but none of them seems to be related to this one. What can I possibly do to fix this?