Is it safe to disconnect an external drive any time?

The difference is Moving Parts

A External Drive has Moving Parts it is a Mechanical HDD that needs it's heads parked before you unplug it.

A Thumb/Portable Drive is just that a Memory Drive and contains no Moving Parts or if you like a Solid State Drive.

With any Mechanical Drive you should always use the Safely Remove Option to Park the Heads and then when you are prompted that it's safe to remove the drive unplug it. With Windows that is always on the Start Bar near the clock though the icon does vary between different versions of Windows.

The reason being that once the heads are parked there is no possibility of the heads touching the moving platters as the power drops down and the heads attempt to park themselves. If you do not use the Safely Remove Option it's just a matter of time till you get a Partition Table Corruption rendering all data on that drive unreadable.

With memory Drives you should still use the Safely Remove Option to prevent any corruption from occurring though this is less important. The reason to use the Safely Remove Option is that the device may be undergoing a write when you unplug it that is not related to something you have wanted written to the device but something that the OS needs to do. If you unplug during a write operation you will get Data Corruption and possibly corrupt the Partition Tables of the device.

1. If you are connecting the drive through an external port, it's an external drive, no matter what else the manufacturer calls it. Thumb drives, key drives, USB drives, portable HDDs, whatever, are all external drives.

2. You should always use the Safely Remove option to remove external drives. This allows Windows to close any open files on the drive, finish any saves to the drive, and in general make sure the drive is ready for removal.

3. If you are unable to get Windows to release the drive ("The drive cannot be removed now, please close all programs that might be using it and try again..."), you can usually release the drive by logging off and logging back on.