What is READ DMA EXT error ?
READ DMA EXT is a disk error reported by ata4.00 utility, most probably a result of unstable voltage supply to your hard disk. When a disk experiences a drop in voltage supply it reset and report errors to the operating system.
Solution to READ DMA EXT error
A quick resort to the problem would be to change your Power supply unity(PSU). If the problem still persists you may need to change your HDD cables as well.
If it is indeed a power supply issue and your disk has suffered electrical failure, backup the data while you can still access it.