I have 2 x HDD that are mirrowed. The HDDs are nearly out of space and I want to replace them with bigger capacity ones. Can I just first add one of the new HDD and wait for it to sink with the old one and then add the other new one and then let it sink with the other new one? Or is it not that easy?
I'm sure OP meant mirrored, and the common way to do this is as a RAID1 array managed by a RAID controller system built into the mobo chipset and BIOS. BUT, as shun_51 says, it's also possible for it to be a dynamic volume composed of two or more HDD units, so knowing which way is important.
OP, IF your system is a RAID1 array, and IF your computer has TWO open ports to add a new pair of drive units, there is a relatively easy way to do this. However, from the sound of your post, this may be more complex than you can do yourself, and you might need a friend or service shop to help out.
Basically, you buy a new pair of HDD's of larger size. At the same time you will need to get a utility software package that does cloning. You can buy a good one called Acronis True Image or some others ones - they do a lot more than just cloning, but you don't need all of that for this job. But customized versions of that software are available to you free for download from the website of some HDD manufacturers. The restriction is that the free Disk Wizard package from Seagate will only make a clone copy TO a Seagate drive (they don't care whose older drive you are leaving behind). Similarly, if you get a new pair of drives from WD, they will let you download Acronis True Image WD Edition to use. So, choose your hard drives and your software together if you want the free software. First you install that utility software on your current drive. Then you install the pair of new drives in your machine. Then you use the built-in RAID management utilities on your mobo to Create a new RAID1 array using those new HDD units, shut down and reboot.
Now you run the cloning software. You must be sure to set it to use the old drive (really, your RAID1 array) as the SOURCE drive, and the new pair as the DESTINATION drive - anything already on the DESTINATION will be destroyed! I assume your existing RAID1 array is working as your C: drive that you boot from. So, you will make sure the clone is a bootable clone, and probably make sure also that its size is NOT the same as the old drive (that may be the default) - you have to manually tell it to use a larger size, likely all of the available space on the new drive. Once you finish making the clone you shut down and disconnect the old two drives. My preference is to re-arrange the data cables so that the new drives are connected to the same mobo ports that the old drives were using. You should be able to boot up from the new drives smoothly so that the only change you can notice is that the C: drive is now much larger.
It is possible that these software cloning tools will not be happy doing their job on a RAID array - I'm not sure. But if that is the case, there is still a slightly more round-about way to get there. Basically it involves breaking the original RAID1 array down to individual drives (as if one of them had failed), cloning to ONE of the new drives, and then converting that new single drive into a new RAID1 array by adding a second new disk to the single first one (as if you were replacing a failed single disk unit from a RAID1 pair).