Very slow external disk read/write speeds on iMac?

what hard drive do you use?

It's your drive limitations. Try SSD drive. That's the best choice for video editing.

Can you tell us the capacity of each drive and how full each is?

There is a problem.

Why are you using RAID enclosures if you are only using single drives (RAID 1)

Also, having a mixture of data protocols on different drives is not a good idea when using these drives for video work. Should be using the same and fastest data protocol drives across the board.

Should just be using USB 3.0, Thunderbolt and FireWire enclosures?

It is possible the FireWire 800 drive could be bringing down the overal transfer speeds of all of the drives.

It is the slowest transfer protocol of all the ones you are using.

You have that FW on the same Thunderbolt bus connection. You could try disconnecting drives to test to see which drive/s combination of drives is slowing the data rate down.

FYI, for video editing USB 3.0, though theoretically much faster than Thunderbolt OR FireWire, still sends data in packets. This is how all the USB protocols work. They all send data in packets or bursts of data. It is not a sustained data stream protocol like Thunderbolt or FireWire.

You may want to consider an all Thunderbolt drive work flow.

Save the USB 3.0 for backups and the FireWire drive as data storage of large project files and libraries. Disconnect/eject these when you are actually working on a video project, so as not to slow down the overall data rate.

Your drives shouldn't be running this slow, especially the USB 3.0 and Thunderbolt drives. Period.

You have some hardware issues going on.

Before SSDs, believe it or not, video was edited using mostly 7200 and 10, 000 RPM hard drives.

Usually with FireWire 400/800 drives.

I create and edit video with Final Cut Express 4 using the internal hard drive with two external FW800 drives connected and working and I am not having the issues you are having.

Although, I am, also, using an older 2009 27 inch screen iMac with 16 GBs of RAM running OS X 10.6.8 Snow Leopard.

Also, I forgot to ask, how much RAM installed in your Mac?

I suspect You have either a hardware and/or hardware/software issues.

It would help us to help you if we could have some more technical info about your iMac.

If you wish, Please download, install and run Etrecheck.

Etrecheck was developed as a simple Mac diagnostic reporting tool by a regular Apple Support forum user and technical support contributor named Etresoft.

Etrecheck is a small, unobstrusive app that compiles a static snapshot of your entire Mac hardware system and installed software.

This is a free app that has been honestly created to provided help in diagnosing issues with Macs running the new OS X 10.9 Mavericks.

It is not malware and can be safely downloaded and installed onto your Mac.

http://www.etresoft.com/etrecheck

Copy/paste and post its report here in another reply thread so that we have a complete profile of your Mac's hardware and installed software so we can all help with your Mac performance issues.

Facinating discussion, when a generous amount of "it should be better" or "hardware issue". The simple fact is Apple is not working hard on supporting external hard drives even their own, a twice the market price of any of the major brands. I'm lucky to see 40 MB/ps R/W on a firewire 800/lightening bolt connections. Every 'Genius' I talked with a the Store agrees that this will be the burden of my brand new iMac. The internal drive runs at speeds 10x or better. I wish I would have know this little bit of info before spending the $3K.

Are your external drives 5400 RPM (which is what a lot of them are)? Your internal is usually 7200 RPM; I returned a Mini once because of the slowness of its internal 5400 RPM drive. It is about 30+% slower.

Check WD website for your drives' specs.

Apple is not working hard on supporting external hard drives even their own

Apple does not manufacture internal or external hard drives. And it is not a computer manufacturer's job to make it compatible with every possible accessory available - rather, it is usually the other way around.