2008 R2 Software Raid 5, writing during resynching

I'm build a software Raid 5 under Windows server 2008 R2 with 4 x 2TB SATA2 (i3-540 4GB)
I launched the process 24h ago, but the resynch progress is 27% only;
- Is there a problem, or is it a normal duration ? Would a 2003R2 raid 5 be faster ?
(I've seen that it could take 4 days, but some people told me it shouldn't take more tha 1 day)
- Can I write on the raid drive during the resynch process ?
Thank you.

Edits :

Actually, I estimate the resynch time to 84 h according to elapsed time and % completed.
I don't know if this calculation is correct, but it seem to match my estimated time.

Value are extracted from test of WD Caviar Green 2TO : http://www.ginjfo.com/dossiers/tests-materiel/composants/disques-durs/caviar-green-2-to-une-capacite-ecolo-hors-norme-20090309?page=3
(Access write 3ms, access read 7ms)

• Read and write at max Sata II :
  4 * 1.820.000 MB at 375 MB/s (Sata II) = 5.4 H


• Access to each cluster of 3 HDD :
  1 * 1.820.000 MB = 28.437.500 64KB clusters at 7ms = 55.3 H (Read simultaneous)


• Write of each parity cluster :
  1 * 1.820.000 MB = 28.437.500 64KB clusters at 3ms = 23.7 H (Write parity)

=> 84.4 H (3.5 days)

If the calculation is correct, it could help some people to estimate the resynch time.