can we efficiently use ros in ubuntu, installed in flash drive ?

edit

Yes, that is definitely possible.

Two things to keep in mind if you want to do this:

1. use a usb stick / flash drive with good read/write performance: some of the more recent USB3 sticks reach (sequential) read speeds of everal tens of MB/sec and have low access latencies. Both are important.
2. use a sufficiently large drive: depending on what you are going to do, packages can take up quite some space, even without considering some of the requirements for data storage (ie: maps, rosbags, etc). This is the same with a regular hdd/sdd, but definitely something to keep in mind.

And it might not be too much of a problem (anymore, or you just don't care), but all flash media have a limited nr of write-cycles. Running an OS (with /tmp and potentially swap space) on a flash drive is a good way to exhaust those quickly (wear levelling will help significantly though).