How necessary is it to eject USB drives before removing them?
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I'm a grad student doing storage research.
Quick answer: if you don't write anything, it should be ok.
Longer answer: this is a bigger issue with storage systems and power loss in general. Storage can be either volatile or non-volatile. If it's volatile, you lose your data when you lose power (e.g., RAM). Non-volatile storage preserves your data even without power, but most mediums are slower to access (e.g., flash, magnetic platters, etc).
Thus, many computers and storage systems, including some flash devices, use a strategy called "write buffering". Basically, if you write a bunch of data, it gets written to some form of volatile storage first (i.e., the "write buffer"), either in the computer or in the device, which is fast, but unsafe if power is lost. Eventually, all the data is copied to non-volatile storage. When you hit eject, you're giving your computer/device a chance to copy all the data to non-volatile storage.
So here are the important points:
1. this is also why it's bad to manually power off your computer instead of doing a normal shutdown
2. it shouldn't matter if you read from your flash device; as long as you don't write anything, it probably doesn't matter if you remove it without ejecting
3. many modern computers will write the data to non-volatile storage on the flash device as quickly as possible, so you're less likely to lose data if you pull without ejecting than if you're using old HW. But it's still better to play it safe.
"2. it shouldn't matter if you read from your flash device; as long as you don't write anything, it probably doesn't matter if you remove it without ejecting
3. many modern computers will write the data to non-volatile storage on the flash device as quickly as possible, so you're less likely to lose data if you pull without ejecting than if you're using old HW. But it's still better to play it safe.