An enlightening remark to begin with. External hard drives are nice but rather vulnarable and error-prone (compared with internal ones). So if it's a bad idea already to have data only on an internal drive (and not on a separate backup), it's a VERY bad idea to have data only on an external drive, especially if they are "very very important". You're sure to lose them then sooner or later.
That being said, I would recommend the following actions:
1. Open the enclosure, disconnect and reconnect the drive.
2. If that fails, mount it (as a slave for example) in your PC or an other enclosure.
3. If that fails also, it's time to either give up or call a professional data recovery company. But that won't be free!
Impossible to predict the outcome.
Hope this helps.
I recently bought this external HITACHI 2.5" hard drive and was using it mainly for backup purposes. Today, while it was on, it fell from the top of my CPU onto my table. And, now when I plug it in, it shows a "Removable Drive L:" in My Computer and when I double click it, it simply says that "The Request Could Not Be Performed Because of an I/O Device Error" and in Device Manager, it shows INITIO INIC-1511 instead of HITACHI.. also when plugged in, the AUTORUN menu juz comes in for one sec and then goes away. Any help/suggestion to recover my data or get this drive wrking again will be highly appreciated as I have my life's work and photographs in this drive and its very very important.